Which animal would you like to have as a pet? Why?
Not 'the' but 'a'.
This is a fish. It's beautiful. It's yellow and red.
This is the parrot. It is yellow and green
This is the tortoise. It is brown
This is the fish. It's beautiful. It's yellow and red
All your sentences are grammatically right, but you can also say:
This is a yellow and green parrot
This is a brown tortoise.
This is a beautiful, yellow and red fish
Great work Pawel!! Ulysse has given you the perfect advice, I wouldn't say anything different to him =)
Keep up the good work!
Blessings - Shanti